1. Field of the Invention
The present invention relates generally to a photomask and a method of controlling the transmittance and phase of light using the photomask. More particularly, the present invention relates to a photomask controlling the transmittance and phase of light using lattice structures wherein a pitch is shorter than a wavelength of the light.
A claim of priority is made to Korean Patent Application No. 2004-4419, filed Jan. 20, 2004, the disclosure of which is incorporated herein by reference in its entirety.
2. Description of the Related Art
High resolution photolithography is generally used in manufacturing highly integrated semiconductor integrated circuits. The efficacy of high resolution photolithography is limited, however, by a mask error enhancement factor (MEEF). Since a photolithography process used to etch a wafer generally requires a critical dimension (CD) distribution that cannot be provided by a conventional photomask due to a prohibitively large MEEF, a photolithography process having a small MEEF is desired.
Many studies have been conducted in order to develop a photolithography process having a small MEEF. In particular, studies have been conducted in order to develop a photolithography process having increased resolution and depth of focus, thus enabling patterns having higher precision to be formed on a wafer. For example, according to one method, increased resolution and depth of focus are attained by forming a lattice structure on a back face of a photomask. According to this method, several methods are employed to achieve a modified illumination effect such as dipole or quadrupole illumination.
Korean Patent Application No. 2003-0067039 discloses a photomask by which a modified illumination effect is obtained using off-axis illumination. Off-axis illumination is obtained by forming a regular and periodic phase lattice on the back face of the photomask. Although modified illumination is obtained by forming the lattice structure on the back face of the photomask, this method fails to control the phase and transmittance.
In order to improve wafer shot uniformity, a different method controls transmittance by minimizing light from zero-order diffraction. Light from zero-order diffraction is minimized by forming lattices on the back face of the photomask and by controlling the density of the lattices. This method, however, only controls the transmittance of light and not the phase. Therefore, when transcribing a complicated variety of patterns on a wafer using an exposure process, pattern defects often occur due to transmittance distribution.
It is advantageous to use a multiphase diffraction optical element instead of a binary phase diffraction optical element to obtain a modified illumination effect because the multiphase diffraction optical element obtains a higher efficiency of images. A method of forming a multiphase diffraction optical element typically comprises forming a stair-shaped structure on the back face of a photomask. However, noise is caused due to limited diffraction efficiency, which has a negative effect on the formation of patterns on a wafer. Accordingly, a photomask capable of removing noise by controlling the transmittance, and also capable of forming a multiphase diffraction optical element is needed.